Today, various display devices have been under active study and development, particularly those based on electroluminescence (EL) from organic materials.
In contrast to photoluminescence (i.e., light emission from an active material due to optical absorption and relaxation by radioactive decay of excited state), electroluminescence (EL) is a non-thermal generation of light resulting from the application of an electric field to a substrate. In the latter case, excitation is accomplished by the recombination of charge carriers of contrary signs (electrons and holes) injected into an organic semiconductor in the presence of an external circuit.
A simple prototype of an organic light-emitting diode (OLED), i.e., a single layer OLED, is typically composed of a thin film of an active organic material, which is sandwiched between two electrodes, one of which needs to be semitransparent in order to observe light emission from the organic layer. Usually, an indium tin oxide (ITO)-coated glass substrate is used as an anode.
If an external voltage is applied to the two electrodes, then charge carriers (i.e., holes) at the anode and electrons at the cathode are injected to the organic layer beyond a specific threshold voltage depending on the organic material applied. In the presence of an electric field, charge carriers move through the active layer and are non-radioactively discharged when they reach the oppositely charged electrode. However, if a hole and an electron encounter one another while drifting through the organic layer, then excited singlet (anti-symmetric) and triplet (symmetric) states (i.e., so-called excitons) are formed. Light is thus generated in the organic material from the decay of molecular excited states (or excitons). For every three triplet excitons that are formed by electrical excitation in an OLED, only one symmetric state (singlet) exciton is created.
Many organic materials exhibit fluorescence (i.e., luminescence from a symmetry-allowed process) from singlet excitons. Since this process occurs between states of same symmetry, it may be very efficient. On the contrary, if the symmetry of an exciton is different from the one of the ground state, then the radioactive relaxation of the exciton is disallowed and luminescence will be slow and inefficient. Because the ground state is usually anti-symmetric, the decay from a triplet breaks symmetry. The process is thus disallowed and the efficiency of EL is very low. Therefore, the energy contained in the triplet states is mostly wasted.
Luminescence from a symmetry-disallowed process is known as phosphorescence. Characteristically, phosphorescence may persist up to several seconds after excitation due to the low probability of the transition, which is different from fluorescence that originates in the rapid decay.
However, only a few organic materials have been identified, which show efficient room temperature phosphorescence from triplets.
Successful utilization of phosphorescent materials holds enormous promises for organic electroluminescent devices. For example, one advantage of utilizing phosphorescent materials is that all excitons (formed by combining holes and electrons in an EL), which are (in part) triplet-based in phosphorescent devices, may participate in energy transfer and luminescence. This can be achieved either by phosphorescence emission itself or by using phosphorescent materials to improve the efficiency of fluorescence process.
In each case, it is important that the light emitting material provides electroluminescence emission in a relatively narrow band centered near selected spectral regions, which correspond to one of the three primary colours (i.e., red, green and blue). This is so that they may be used as a coloured layer in an OLED.
As a means for improving the properties of light-emitting devices, there has been reported a green light-emitting device utilizing the emission from ortho-metalated iridium complex. (Ir(ppy)3: tris-ortho-metalated complex of iridium (III) with 2-phenylpyridine (ppy). Appl. phys. lett. 1999, vol. 75, p. 4.
Thus, US 2005214576 (SERGEY LAMANSKY ET AL.) 29 Sep. 2005 discloses emissive phosphorescent organometallic compounds useful in the fabrication of organic light emitting devices, which are exemplified by the following: platinum(II)(2-phenylpyridinato-N,C2′)(acetyl acetonate) [Pt(ppy)(acac)]; platinum(II)(2-(p-tolyppyridinato-N,C2′) (acetyl acetonate) [Pt(tpy)(acac)]; platinum(II)(7,8-benzoquinolinato-N,C3′) (acetyl acetonate) [Pt(bzq)(acac)]; platinum(II)(2-benzylpyrinato-N,C2′) (acetyl acetonate) [Pt(bzpy)(ocac)]; platinum(II)(2-(2′-thienyl)pyridinato-N,C3′) (acetyl acetonate) [Pt(thpy)(acac)]; platinum(II)(2-(2′-(4′,5′-benzothienyl)pyridinato-N,C3′) (acetyl acetonate) [Pt(btp)(acac)]; platinum(II)(2-(4′,6′-difluorophenyl)pyridinato-N,C2′) (acetyl acetonate) [Pt(4,6-F2 ppy)(acac)]; platinum(II)(2-(4′,5′-difluorophenyl)pyridinato-N,C2′) (acetyl acetonate) [Pt(4,5-F2 ppy)(acac)]; and platinum(II)(2-(4′,5′-difluorophenyl)pyridinato-N,C2) (2-picolinato) [Pt(4,5-F2 ppy)(pico)].
WO 2005/117159 (CDT OXFORD LIMITED) 8 Dec. 2005 discloses a metal complex for emitting light represented by formula I, which is shown below:    M-Lwherein M is a metal, L is a ligand and L comprises Ar that is a substituted or unsubstituted heteroaryl ring, which contains at least one phosphorus atom. This suggests that L is preferably a bidentate ligand such as bipyridyl.
WO 2005/117160 (CDT OXFORD LIMITED) 8 Dec. 2001 discloses a charged metal complex useful for light emitting devices. The charged metal complex may be fluorescent or phosphorescent, which contains metal M and coordinate ligand L. Suitable metals M include lanthanide metals, d-block metals and metals forming fluorescent complexes. Further, ligand L may be monodentate, bidentate or tridentate.
SPROUSE, S., et al. Photophysical effects of metal-carbon a bonds in ortho-metalated complexes of Ir(III) and Rh(III). J. Am. Chem. Soc. 1984, vol. 106, p. 6647-6653. disclose dichloro-bridged dimmers of the type [M(L)2Cl]2, wherein L is 2-phenylpyridine (ppy) or benzo[h]quinoline (bzq) and M is Rh(III) or Ir(III). The above reference teaches that the ortho-metalated ligands exhibit considerably higher spectroscopic effects and lower energy charge transfer transitions compared to Rh(III) and Ir(III) complexes of 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen).
SLINKER, Jason D., et al. Efficient yellow electroluminescence from a single layer of a cyclometalated iridium complex. J. Am. Chem. Soc. 2004, vol. 126, p. 2763-2767. disclose a charged iridium complex, [Ir(ppy)2-(dtb-bpy)]+(PF6)−, and its use as a multifunctional cyclometalating ligands. The charged iridium complex contains three ligands, wherein two cyclometalating ligands (ppy: 2-phenylpyridine) are chosen to coordinate the iridium metal center to further increase the ligand field splitting energy. Further, the third ligand, 4,4′-di-tert-butyl-2,2′-dipyridyl (dtb-bpy), ensures redox reversibility, decreases self-quenching and enhances device characteristics.
LEPELTIER, Marc, et al. Synthesis, structure and photophysical and electrochemical properties of cyclometallated iridium(III) complexes with phenylated bipyridine ligands. Eur. J. Inorg. Chem. 2005, p. 110-117. disclose a series of cationic diminoiridium(III) complexes, [Ir(ppy-N,C)2(L-N,N)](PF6)(Hppy=2-phenylpyridine, L=4,4′-u2dpbpy, 4,4′-Me2dpbpy, 4,4′-Me2pbpy, 4,4′-Me2bpy), and their photophysical and electrochemical properties.
SLINKER, Jason D., et al. Green electroluminescence from an ionic iridium complex. Appl. phys. lett. 2005, vol. 86, p. 173506. disclose green fluorescence from an ionic iridium complex, [Ir(F-mppy)2(dtb-ppy)]+(PF6−), wherein F-mppy is 2-(4′-fluorophenyl)-5-methylpyridine and dtb-bpy is 4,4′-di-tert-butyl-2,2′-bipyridine.
EVANS, Rachel C., et al. Coordination complexes exhibiting room-temperature phosphorescence: Evaluation of their suitability as triplet emitters in organic light emitting diodes. Coordination Chemistry review. 2006, vol. 150, p. 2093-2126. disclose several iridium(III) complexes containing cyclometalated ligands such as    4-(4′-chlorophenyl)-6′-phenyl-2,2′-bipyridine (clpby),    4′-(4-carboxyphenyl)-6′-phenyl-2,2′-bipyridine (cpbpy),    4,4′-dibutyl-2-2′-bipyridine (dbbpy),    4-(4-hydroxyphenyl)-6′-phenyl-2,2′-bipyridine (hpbpy) or    4′-(4-tolyl)-6′-phenyl-2,2′-bipyridine and its derivatives.
However, since the above light-emitting materials of the prior art do not display pure colours, i.e., their emission bands, which are generally limited to green, are not centered near selected spectral regions (corresponding to one of the three primary colours—red, green and blue), the range that they can be applied as OLED active compound is narrow. It has thus been desired to develop light-emitting materials, which are capable of emitting light with other colours, especially in the red region.
Efficient and long-lived red-light emitters with good colour coordinates are a recognized current shortfall in the field of organic electroluminescent devices.